Recovery boilers are fueled with waste liquor generated in connection with pulp manufacture containing various sodium salts, mainly sodium carbonate and sodium sulfate, in addition to organics and water. These salts form a salt bed on a furnace floor during boiler operation. The bed is at least partly molten so that molten salt flows continuously from the bed to smelt spouts through smelt spout openings located on a lower part of the furnace, and further to a dissolving tank.
The smelt spout openings are typically placed at a level higher than the furnace floor so that the salt bed on the floor is at least 200-250 mm thick.
Recovery boilers have become gradually larger so that modern recovery boilers have a floor area of 150-300 m2. In addition, modern recovery boilers are typically equipped with a so called decanting floor, with floor tubes sloping downwards from a back ball, where the smelt spout openings typically are, towards the centerline of the floor so that the lowest elevation extends several meters, even over eight meters, from the back wall. The salt bed residing on the floor is hence considerably thicker at the centerline than in the proximity of the back wall.
These factors—the increase in recovery boiler size, the use of decanting floor as a standard design instead of flat floors or floors sloping towards the back wall—have together led to the current situation in which the remaining salt bed during the outage in a modern recovery boiler is large both in thickness and volume; in the largest boilers the salt bed thickness at the floor centerline is 0.5-0.7 m and the total volume of the bed is over 100 m3.
When the furnace floor is inspected, the floor needs first to be made free from salt (smelt) remaining on the floor. Conventionally, the recovery boiler furnace floor is cleaned as follows:                1. During washing of superheaters, hot water is pumped to furnace floor through nozzles installed in primary air port openings. Water also rains or flows to the floor typically from soot blowers used in superheater washing and possibly from wash water sprays used in superheater washing or wall washing. Water on the floor flows out of the furnace as an overflow through smelt spout openings to smelt spouts and further to the dissolving tank. Water in the dissolving tank can be partly recirculated back and further used in furnace floor washing.        2. As soon as the superheater washing is completed, the water pool on the floor is emptied using for example a vacuum truck. And, the salt remaining on the floor is removed mechanically by chopping the salt bed to pieces that are small enough so that they can be piled on top of each other in the middle of the furnace or moved away from the furnace for example to a vacuum truck.        
The used method is labor-intensive, time consuming, expensive and risky. The outcome of the washing is often so poor that a large part of the salt remaining on the floor has to be removed mechanically. The floor cleaning after the washing may thus take even several days. The time spent on floor cleaning often extends the whole mill outage and accordingly reduces the mill production, so the costs due to this loss of production can be several millions of euros. Mechanical cleaning with chopping the salt may also damage the floor tubes, in which case additional outage time is needed for repairing the damages.
Due to the risks associated with mechanical cleaning the floor washing is performed in certain mills by using high pressure washers which are installed in smelt spout openings or burner openings. The high pressure washers employ considerably high pressure, 80-100 MPa (800-1000 bar) or even 250-300 MPa. The pressures used are of the same order of magnitude as those used in steel cutting machines, so also the use of high pressure washers can damage the floor or wall tubes.